In recent years, many new technologies related to Liquid Crystal Displays (LCD) have come forth, and among others, Liquid Crystal on Silicon (LCOS) is a hot technology. LCOS relates to a new reflective micro LCD projection technology. To form an LCOS structure, transistors are grown on a silicon substrate; a drive panel (also referred to as Complementary Metal-Oxide-Semiconductor-LCD (CMOS-LCD)) is fabricated using a semiconductor process; the transistors are flattened by polishing technology and plated with aluminum to act as micro mirrors; thus a Complementary Metal-Oxide-Semiconductor (CMOS) substrate is formed; then the CMOS substrate is jointed with an upper glass substrate having transparent electrodes, and liquid crystal is injected into the structure. An encapsulation test is then performed.
Compared with conventional LCD and Digital Light Processing (DLP) technologies, LCOS has the following technical advantages: a) high light utilization efficiency: LCOS is similar to LCD technology, and mainly different from LCD in that LCOS is a reflective imaging system, such that the light utilization efficiency may reach 40% or more which is equivalent to the light utilization efficiency of DLP, while the light utilization efficiency of transmissive LCD only reaches about 3%; b) small volume: LCOS may integrate periphery circuits such as driver ICs to a CMOS substrate completely, so as to reduce the number of periphery ICs and the encapsulation cost, and decrease the whole volume; c) high resolution: since the transistor and the driver circuits of an LCOS are both fabricated in a silicon substrate and located under the reflective surface, they don't occupy surface area, and only pixel gaps occupy opening area, while Thin Film Transistors (TFTs) and wires of a transmissive LCD both occupy the opening area, so that both the resolution and the opening ratio of LCOS are higher than those of transmissive LCD; d) more mature manufacturing technology: the manufacturing of LCOS may be divided into Front of Line (FL) semiconductor CMOS manufacturing and End of Line (EL) liquid crystal panel jointing and encapsulating. There have been mature designing, simulating, fabricating and testing technologies for the FL semiconductor CMOS manufacturing, and now the product yield has reached above 90% with a very low cost; as for the EL liquid crystal panel jointing and encapsulating, although the yield is only 30% at now, since the manufacturing of liquid crystal panel has been developed rather maturely, the yield may be increased more rapidly than that of digital micromirror device (DMD) in theory. As a result, LCOS has more chances to be the mainstream technology than DLP. Therefore, LCOS technology has a bigger potential in application markets of digital camera, digital video camera, projector, monitor, large size TV and mobile telephone.
In LCOS technology, each pixel switch circuit consists of a Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) and a capacitor. In conventional processes, the capacitor occupies half of the whole pixel area. As the circuit area decreases, the capacitor area also decreases. This will increase refresh rate in practical use. In order to increase capacitance, U.S. Pat. No. 6,437,839 discloses an LCOS pixel with multiple capacitors, the structure of which is shown in FIG. 1, and formed by the following steps: a diffusion region is formed on a substrate 40 as a top electrode of a first capacitor; an oxide layer is formed on the diffusion region as a dielectric layer 22b of the first capacitor; a first polysilicon layer 26 is formed on the dielectric layer 22b as a common electrode of two capacitors; a second oxide layer is formed on the first polysilicon layer 26 as a dielectric layer 24b of a second capacitor; a second polysilicon layer is formed on the second oxide layer as a top electrode 24a of the second capacitor; an insulation layer 42 is formed on the above structure; and an interconnection structure and an micromirror layer is formed on the above structure. As shown in FIG. 1, the capacitance is increased in the improved structure, but the processes are also increased, resulting in increased processing cost.
At present, each micromirror on the substrate surface serves as a display pixel, and each display pixel has a switch circuit. A pixel switch (MOSFET) and a capacitor must be designed in the area of a pixel. In order to lower the display refresh rate, the capacitance needs to be as large as possible. However, as restricted by the pixel area, if the capacitor occupies the pixel area in too high proportion, the performance of the switch circuit may be affected inevitably. For example, since the design area of the MOSFET is decreased, the insulation performance of the MOSFET may be affected and current leakage may occur.